Closed Lake System Research Articles

Depositional controls in an ancient, closed lake system: A high-resolution and multi-scalar case study from the Yacoraite Formation (Salta Basin, Argentina)

The Yacoraite Formation (Cretaceous–Paleogene) is a mixed carbonate–siliciclastic microbialite-bearing lacustrine succession deposited in the intra-continental Salta rift basin (Argentina). In the northern sub-basin of Tres Cruces, the spectacular exposures and high lateral continuity of the outcrops provide a suitable setting to investigate and reconstruct the paleoenvironments of deposition and the resulting facies architecture of this closed, saline lake system. The Yacoraite Formation deposited in a predominantly shallow-water ramp-like lake system, characterized by eighteen main facies, classified within four main facies associations, each defining a particular depositional environment along the lake profile. From proximal (shallow) to distal: i) Palustrine Facies Association; ii) Littoral Facies Association; iii) Sub-littoral Facies Association; and lastly iv) Profundal Facies Association. In the Yacoraite paleo-lake extensive fringing mud-flat environments surrounded the lake margins, while the littoral marginal areas hosted extensive wave-dominated oolitic–bioclastic grainstones forming shoals and bars parallel to the lake margins and provided a sheltered environment in the back-shoals that allowed prolific microbialite development. In proximity to river inputs, shallow-water deltas and shoreline sandstones accumulated in the littoral margins. In the sub-littoral zone, mud-supported carbonates such as ostracod wackestones and mudstones deposited below the fair-weather wave base, and in the profundal settings deep-water organic-rich shales deposited. The Yacoraite paleo-lake evolved from a relatively stable and perennial lake system (lake stage 1) into a rapidly fluctuating ephemeral lake system (stage 2), as a result of progressively changing environmental and climatic conditions toward more arid settings. Climate control is also critical in the facies architecture, with the development of short-term cyclicity, represented by meter-scale transgressive–regressive (T–R) cycles being the result of lacustrine expansion–contraction cycles. The regressive hemicycles represent shallowing-upward succession of sub-littoral and littoral carbonate-dominated facies, whereas the transgressive hemicycles are mostly represented by sub-littoral and profundal mudstones and shales. Stacking of short-term cycles resulted in the development of medium- and long-term cyclicity at the scale of tens of meters, which represent the long-term evolution of the lake system. Lastly, a comparison with two well-documented closed lake systems highlights the role of allogenic factors acting at a regional scale (e.g., climate and tectonics) in controlling the lake-basin type, facies association and the resulting architecture. Factors at the lake-basin scale (e.g., lake margin profile, bathymetry) modulate the expression of facies and cyclicity. Ultimately, the characterization of the kilometer-scale outcrops of the Yacoraite paleo-lake provides a reference analog study to support the characterization of similar depositional systems and reservoirs of other ancient, closed lake systems, including the Pre-Salt Carbonates of the South Atlantic.

Variations of water quality indicators in closed tropical freshwater systems towards selected climatic and anthropogenic disturbances

Inland waters are nowadays becoming hotspots of endangerment. Disturbance is any change in set ofenvironmental conditions which alter regular ecosystem properties that may be natural or anthropogenic.Disproportionate enrichment due to several point and non-point sources has affected water quality and natural trophic status of inland waters where climate change can be an indirect factor. Lakes are significant indicators of climate change and store house of information about changes in catchment characteristics. Climate change may affect tropical lakes more comparatively. Water temperature affects a number of lake properties that influence the wellbeing of lake ecosystem. Increasing temperature and deviation of normalrainfall pattern have been evidently studied for changes in chemical composition of lakes. The presentstudy is an effort to investigate the effects of change which can be direct or indirect occurring over a periodof time in a closed system (Purohitokatalab) of Udaipur, Southern Rajasthan with reference to alteration inits water quality subjected to several tourist activities within recent years. Such isolated lakes lack selfcleansing mechanism due to absence of strong flow. The values and alterations of parameters i.e. pH, TDS,conductivity, turbidity, total hardness, chlorides, sulphates, nitrates, phosphates, DO, BOD indicate thequality of lake water. Several samples of water from the periphery of this closed lake system were collectedduring year 2017 and 2021 and these water quality parameters were analysed. Results indicated high variation in the N:P ratio within the freshwater body. It also demonstrated increasing trends in nitrateconcentrations, alkalinity and biological oxygen demand over the time period with change in temperature and rainfall variations. Major changes in the catchments characteristics and measures for its management were also discussed to avoid further deterioration of the closed natural ecosystem. Here future monitoring of water quality indicators is crucial to assess the impact of climate change on such tropical closed systemsas it is a potential threat stimulating the precursors of eutrophication and overall alteration of ecologicalstate of lakes.

The Holocene lake-evaporation history of the afro-alpine Lake Garba Guracha in the Bale Mountains, Ethiopia, based on δ18O records of sugar biomarker and diatoms

AbstractIn eastern Africa, there are few long, high-quality records of environmental change at high altitudes, inhibiting a broader understanding of regional climate change. We investigated a Holocene lacustrine sediment archive from Lake Garba Guracha, Bale Mountains, Ethiopia, (3,950 m asl), and reconstructed high-altitude lake evaporation history using δ18O records derived from the analysis of compound-specific sugar biomarkers and diatoms. The δ18Odiatom and δ18Ofuc records are clearly correlated and reveal similar ranges (7.9‰ and 7.1‰, respectively). The lowest δ18O values occurred between 10–7 cal ka BP and were followed by a continuous shift towards more positive δ18O values. Due to the aquatic origin of the sugar biomarker and similar trends of δ18Odiatom, we suggest that our lacustrine δ18Ofuc record reflects δ18Olake water. Therefore, without completely excluding the influence of the ‘amount-effect’ and the ‘source-effect’, we interpret our record to reflect primarily the precipitation-to-evaporation ratio (P/E). We conclude that precipitation increased at the beginning of the Holocene, leading to an overflowing lake between ca. 10 and ca. 8 cal ka BP, indicated by low δ18Olake water values, which are interpreted as reduced evaporative enrichment. This is followed by a continuous trend towards drier conditions, indicating at least a seasonally closed lake system.

Mid-Holocene palaeohydrochemistry and palaeohydrology of Yamdrok Yumtso, southern Tibetan Plateau, reconstructed from δ18O and δ13C of fossil shells of the gastropod Radix auricularia

Palaeohydrochemical and palaeohydrological changes of lakes have seldom been reconstructed from the fossil shells of the gasropod Radix auricularia, which is a new, and potentially high-resolution environnmental archive. We conducted a geochemical and stable isotope study of the shells of Radix from the exposed fluvio-lacustrine sediments near Lake Yamdrok Yumtso in the southern Tibetan Plateau. Our aims were to determine the sedimentary environment, palaeo-lake hydrochemistry and hydrological status. AMS 14C and OSL dating indicates that a lake-level stage of Yamdrok Yumtso higher than that of today occurred during ~4.7–1.2 cal. kyr BP. Results of Sr/Ca, δ13C and δ18O analysis of the fossil shells of Radix auricularia indicate that the lake-level fluctuations were mainly controlled by changes of the Indian Summer Monsoon; decreasing evaporation during the mid- to late-Holocene was also responsible. In addition, based on the geochemical relationship between Radix sp. shells and the ambient water in lakes, the values of δ18OPalaeo-water and Sr/CaPalaeo-water reconstructed using the fossil shells of Radix auricularia are −8.2‰ to −5.1‰ and 0.0012 to 0.0057, respectively. Further, based on the values of δ18Oshell, together with geomorphological evidence, we infer that Yamdrok Yumtso was a closed lake system, and we estimate its possible extent during the interval of high lake-level. In addition, we speculate that the water level of Yamdrok Yumtso at this time exceeded 4448.9 m a.s.l., but was less than 4451 m a.s.l., and that the major separation of various components of the Yamdrok Yumtso system occurred after 1.2 kyr BP.

Isotope hydrology model and stable isotopes in sediment records from Balkan lakes

<p>Isotope mass balance in lake systems is strongly correlated with several climatic factors such as temperature, evaporation, precipitation and air moisture. On the other hand, the sedimentary budget of lake basins driven by climate, tectonic and/or human impact is an essential pool of environmental records. Precipitation, springs and lake water δD and δ18O isotope data were used in order to understand the key factors for the hydrological balance of Balkan lakes in West Macedonia. In general, it is concluded that the open lake Ohrida and the semi-closed lake Kastoria are more buffered hydrological as karst systems and less sensitive to evaporation effect, in contrast to the closed lake system of Prespes that present a strong dependence on climate seasonality. Based on oxygen isotopes in bulk sediments, it is concluded that in Kastoria lake basin an increased run-off on the land surface, probably under a more humid period, in 2.4 kyr BP disturbed the transition to a drier regime from mid/late Holocene to present.</p>

Reviews and syntheses: How do abiotic and biotic processes respond to climatic variations in the Nam Co catchment (Tibetan Plateau)?

Abstract. The Tibetan Plateau (TP) is the largest alpine plateau on Earth and plays an important role in global climate dynamics. On the TP, climate change is happening particularly fast, with an increase in air temperature twice the global average. The particular sensitivity of this high mountain environment allows observation and tracking of abiotic and biotic feedback mechanisms. Closed lake systems, such as Nam Co on the central TP, represent important natural laboratories for tracking past and recent climatic changes, as well as geobiological processes and interactions within their respective catchments. This review gives an interdisciplinary overview of past and modern environmental changes using Nam Co as a case study. In the catchment area, ongoing rise in air temperature forces glaciers to melt, contributing to a rise in lake level and changes in water chemistry. Some studies base their conclusions on inconsistent glacier inventories, but an ever-increasing deglaciation and thus higher water availability have persisted over the last few decades. Increasing water availability causes translocation of sediments, nutrients and dissolved organic matter to the lake, as well as higher carbon emissions to the atmosphere. The intensity of grazing has an additional and significant effect on CO2 fluxes, with moderate grazing enhancing belowground allocation of carbon while adversely affecting the C sink potential through reduction of above-surface and subsurface biomass at higher grazing intensities. Furthermore, increasing pressure from human activities and livestock grazing are enhancing grassland degradation processes, thus shaping biodiversity patterns in the lake and catchment. The environmental signal provided by taxon-specific analysis (e.g., diatoms and ostracods) in Nam Co revealed profound climatic fluctuations between warmer–cooler and wetter–drier periods since the late Pleistocene and an increasing input of freshwater and nutrients from the catchment in recent years. Based on the reviewed literature, we outline perspectives to further understand the effects of global warming on geodiversity and biodiversity and their interplay at Nam Co, which acts as a case study for potentially TP-level or even worldwide processes that are currently shaping high mountain areas.

The shoreline distribution and degradation of tilapia carcasses, Salton Sea, California: Taphonomic implications

The Salton Sea is a hypersaline, closed lake system in southern California, USA. Episodic fish kills led to bloating and floating fish bodies that were driven onto the shoreline by wind shear. The documented taphonomic destruction of the stranded fish carcasses includes the distribution of the fish parts after desiccation and disarticulation mainly by wave-induced transport with possible minor component of wind transport on the upper shoreface. Carcass decay is divided into five taphonomic categories 1 through 5. Category 1 represents whole pliable fish, recently washed onto the shoreline while category 5 incorporates longer-term destruction into ultimately single, isolated skeletal elements. High water, strand line deposits are delineated by the presence of categories 3 through 5 mixtures. The general trend, in 2012 was observed to be considerable bone disarticulation away from the shoreline because categories 1 and 2 were present in the swash zone; the influx of fresh fish carcasses recharged the shoreline. During 2013, the absence of new 1 and 2 categories arriving to the shoreline enhanced wave action to further break down categories 3 and 4 ultimately into category 5. In addition, isolated bone elements segregate according to density differences, typically on low-gradient beaches. Wave and wave-induced current action coupled with beach topography controls distribution of the various fish bones. As an actualistic model, the Salton Sea implies that shore-parallel complete to partial fish carcass concentrations are more shoreward and isolated skeletal elements more distal.

Mineralogy, geochemistry and genesis of the modern sediments of Seyfe Lake, Kırşehir, central Anatolia, Turkey

Seyfe Lake (Kırşehir, Turkey) is located within a depression zone extending along a NW–SE-trending fault in central Anatolia. Evaporite and carbonate sediments occur at the bottom of the lake which is fed by high-sulfate spring and well waters circulating N–S through salt domes. The recent sediments of Seyfe Lake are deposited in delta, backshore, beach, mud-flat and shallow lake environments. In the mud-flat environment, calcite, gypsum, halite, and thenardite are associated with fine-grained detrital sediments. Sediments from the margin to the lake center are distributed as calcite, gypsum and halite±thenardite, yielding an annular distribution pattern. An increase in Na2O, SO3, and S, and a decrease in CaO toward the lake center are due to sediment distribution. On the other hand, a positive correlation of SiO2 with MgO, K2O, Na2O, Al2O3, and Fe2O3+TiO2 is attributed to the presence of smectite, illite and feldspar. In addition, a positive correlation of Sr and Ba with CaO is related to the amount of gypsum in the sediments. Strontium is associated with in situ gypsum crystals; it increases in the intermediate and central zones of the lake as a result of a relative increase in salinity toward the lake center. The association of Sr with gypsum in the sediments suggests that Ca and Sr were derived from Sr-bearing evaporites and their carbonate host rocks, which were the likely aquifers for the brine. The S- and O-isotopic compositions of sulfate crystals range from +19.1‰ to +21.7‰ and from +16.9‰ to +20.9‰ SMOW, respectively, suggesting precipitation in a closed lake system. A relative increase of oxygen and sulfur isotope ratios toward the lake center suggests dissolution of gypsum in the host rock, with contributions from circulating groundwater and sulfate reduction (possibly by bacterial reduction). 87Sr/86Sr isotope ratios range from 0.707286 to 0.707879, suggesting a non-marine Oligo-Pliocene evaporitic host rock source for precipitation in Seyfe Lake. The concentration of Sr- and S-isotope ratios in the gypsum crystals indicates formation by precipitation/recrystallization from brine rather than from seawater.

The Late Pleistocene evolution of palaeo megalake Bungunnia, southeastern Australia: A sedimentary record of fluctuating lake dynamics, climate change and the formation of the modern Murray River

The ephemeral lacustrine carbonates of the Bungunnia Limestone, deposited in palaeo megalake Bungunnia, preserve a detailed record of Pleistocene palaeoenvironmental and palaeoclimatic change in southeastern Australia. Five distinct lake shorelines (Lake Levels 0–4) are visible on digital elevation models, ranging in elevation from around 70m to 30m above sea level. The Bungunnia Limestone is preserved on the three lowest terraces ranging over 20m in elevation (Lake Levels 2–4). On the two higher of these terraces Bungunnia Limestone contains ooids and stromatolites and is calcite- and aragonite-dominated whereas on the lower terrace level Bungunnia Limestone is dolomite-, gypsum- and magnesite-dominated and preserves tepee structures and mud-cracks. Thus, lacustrine conditions are interpreted to have become increasingly saline and evaporitic over time reflecting an overall trend to more arid climatic regimes. The highest lake level (Level 0 at around 70m above sea level) appears to relate to a period of significantly increased rainfall that allowed the lake to overflow for the first time, marking the initiation of the modern Murray River. Subsequently, the lake fluctuated between: (1) a steady-state open lake system where water was overflowing across the Padthaway High spillpoint and active gorge cutting was proceeding, and (2) a closed lake system with no overflow that resulted in carbonate-precipitating conditions. During (1), shoreline erosion was occurring within the basin providing the terraces on which the Bungunnia Limestone carbonates were deposited during (2). In this, (1) is likely to correspond with significantly wetter Late Pleistocene interglacial and (2) with arid glacial conditions. The formation of the modern Murray River and the demise of palaeo megalake Bungunnia were therefore consequences of a combination of gorge cutting and increasing aridity resulting from interglacial–glacial climate oscillation superimposed on significant overall climatic change.

Sequence characteristics of foreland basin and its difference from those of other types of basin

Based on the case history study of sequence stratigraphic framework, sequence model, and the controlling factors on sequence formation in the southern Junggar foreland basin, a contrasting comparison of the characteristics among the foreland basin, the faulted basin, and the sag basin is made in the aspects of sequence architecture, filling style, the controlling factors on sequence development, the migrations of basin center and depocenter, etc.. The current study results show that there are major differences and these are documented as follows. (1) The accommodation space in foreland basin is mainly created by the subsidence caused by thrusting and napping in the foothill belt, resulting in progradational-dominated parasequence sets in the foothill area and retrogradational parasequence sets in the ramp region. (2) The accommodation space in an open-lake system in faulted basin is mainly created by the activities of the basin margin faults; thus, tectonic sequences are formed. However, in the closed-lake system, the subaqueous accommodation is mainly controlled by climatic-influenced lake-level fluctuations, and climatic sequence is formed. (3) In sag basin, a closed-lake system is easily formed due to its generation often within the cratons and isolation from the sea, and its accommodation space creation is mainly controlled by climatic lake-level fluctuations; thus, the sequence architecture in sag basin is similar to that formed on the passive continental margin.

Laser Ablation Inductively Coupled Plasma Mass Spectrometric Analyses of Base Metals in Arctic Char (Salvelinus alpinus) Otoliths Collected from a Flooded Base Metal Mine

Otoliths from arctic char recovered from the water body formed from an abandoned open-pit nickel-copper mine contain a trace element record related to the geology of the immediate watershed, past mining activity in the area, and the fish's diet. Laser ablation inductively coupled plasma mass spectrometric analyses across the annular structure of the otoliths detected trace amounts of nickel, copper, and chromium believed to be related to the metal-bearing, mafic-ultramafic minerals in the pit. Oscillatory strontium, barium, and zinc profiles may reflect changing water temperature, diet, or fish metabolism. Lead was detected in very low concentrations and may be related to anthropogenic influence. This closed lake system provides a unique opportunity to study an introduced exotic species in a setting where neither migration nor recruitment have been possible. The fish have successfully occupied the lake and continue to breed despite the influence of the surrounding rocks and local contamination. The chemical record retained within otoliths provides a method of monitoring trace elements affecting fish on a yearly basis and may be regarded as a useful assessment tool for examining the exposure of wild organisms to trace elements.

The Lago Cardiel Basin, Argentina (49°S): Origin and evolution revealed by high-resolution multichannel seismic reflection studies

A multichannel, high-resolution seismic reflection survey was conducted with an airgun source in the closed lake system of Lago Cardiel, Argentina (49°S). As part of an ongoing study of interhemispheric correlations in climate change, it was intended to determine the maximum age and depth of the lake sediments and better understand the origin and evolution of the lake basin. In agreement with previous single-channel, 3.5-kHz surveys, six major seismic sequences throughout the lake’s subsurface were mapped. The stronger seismic source of this study allows the deepest sequence (Sequence VI) to be interpreted as folded Cretaceous marls with a highly eroded surface. Further examination of this sequence reveals various stratigraphic signatures and mechanisms of the generally east–west orientation of tectonic compression. The nature and continuity of Sequence VI do not indicate a volcanic or impact-related origin of the lake. The original basin has a maximal depression of 154 ms (∼114 m) deep beneath the modern lake surface and is overlain by an alluvial fan (Sequence V) of likely Pleistocene age in the western portion and late Quaternary lacustrine sediments (Sequences I–IV) everywhere else. The data confirm that southern Patagonia was dominated by extremely contrasting climatic conditions during the Quaternary.

Crater lakes of the Pali Aike Volcanic Field as key sites for paleoclimatic and paleoecological reconstructions in southern Patagonia, Argentina

Sedimentary records from crater lakes are of major scientific interest because they provide continuous high-resolution climatic and environmental archives. From a limnogeological survey of crater lakes performed in the Pali Aike Volcanic Field (52°S, southeastern Patagonia, Santa Cruz, Argentina), two deep crater lakes have been recognized: Laguna Potrok Aike (100 m water depth) and Laguna Azul (56 m water depth). Physico-chemical analyses of these closed lake systems demonstrate that Laguna Azul has a dimictic and thermally stratified freshwater body, whereas Laguna Potrok Aike is a subsaline polymictic lake. Both have an oxygen-rich water column from top to bottom. Laguna Potrok Aike in particular is enriched in Na, P, and Cl. The morphometry suggests that Laguna Azul is of Holocene age, whereas the potential sediment infill of Laguna Potrok Aike may comprise 250 m to a mid-Pleistocene age (770 ka). Several aerial and subaquatic lake level terraces at Laguna Potrok Aike point to lake level fluctuations triggered by prior hydrological changes. Although fine-grained sediments of both lakes are not varied, they may eventually provide a detailed terrestrial record of past environmental and climatic variations for this southern mid-latitude region.

Paleohydrology of an Upper Aptian lacustrine system from northeastern Brazil: Integration of facies and isotopic geochemistry

The Codó Formation records the initial evolutionary stages of an intracontinental rift system formed along the Brazilian equatorial margin in the late Aptian. Deposits of this unit exposed on the eastern margin of the Grajaú Basin include gypsum, bituminous black shales and limestones. These lithologies were formed in a low energy, well stratified, anoxic and hypersaline lake system developed in a dominantly arid/semi-arid climate. This lacustrine succession is internally organized into three categories of shallowing-upward cycles, with the first- and second-order cycles being related to seismic activity associated with fault reactivations, and the third-order cycles recording climatic fluctuations. Studies emphasizing petrography and analysis of the geochemical tracers Fe, Mg, Sr, Mn, Na and Ca helped to identify the sedimentary facies that kept a primary signal, which were thus appropriate for isotopic investigations aiming paleoenvironmental and paleohydrologic reconstructions. The results of this study revealed a wide distribution of dominantly low carbon and oxygen isotope values in carbonates, ranging from − 5.69‰ to − 13.02‰ and from − 2.71‰ to − 10.80‰, respectively. This paper demonstrates that at least in the particular case of oxygen, the isotope ratios vary according to seismically-induced shallowing-upward cycles, with values in general lower at their bases, where central lake deposits dominate, and progressively higher upward, where marginal lake deposits are more widespread. In addition to confirming a depositional signature for the analysed samples, this behavior allowed the development of a seismic-induced isotope model. The lighter isotope ratios appear to be related to flooding events promoted by subsidence, which resulted in the development of a perennial lake system, while heavier isotope values are related to ephemeral lake phases favored by uplift and/or increased stability. Furthermore, the results show that a closed lake system dominated, as indicated by the overall good positive covariance (i.e., + 0.42 to + 0.43) between the carbon and oxygen isotopes, though open phases are also recorded by negative covariance values of − 0.36. During closed phases, the δ 18O displayed the highest range of variation (i.e., − 3.63‰ to − 4.89‰) due to increased residence time, while this variation was low (i.e., − 0.09‰ to − 1.87‰) during open lake phases, when there was a balance in the water isotope composition maintained by continuous basin inflow.

Accessing limnological change and variability using fossil diatom assemblages, south‐east Australia

AbstractFloodplain wetlands accumulate river‐borne sediments that include mixed assemblages of allochthonous and autochthonous diatoms as fossils. These assemblages have been used in river floodplain wetlands and reservoirs to quantitatively reconstruct salinity, pH and nutrients and to qualitatively infer connectivity and turbidity over periods spanning decades to millennia. High sedimentation rates in some sites have permitted sub‐annual temporal resolution; however, annual to decadal resolution is more usual. The establishment of chronologies for these sequences is often difficult owing to the substantial input of fluvially borne210Pb, the high spatial variability in the earliest detection of exotic pollen markers and the inaccuracy of radiocarbon approaches in dating sediments younger than 500 years. Other complexities arise from the difficulty of differentiating the influence of co‐variables in accord with the river continuum concept and identifying shifts driven by hydroseral influences independent of changes to the fluvial system. Caution is also needed in inferring lotic change from a record accumulating in lentic systems.Nevertheless, substantial increases in salinity (lower Snowy, lower and middle Murray), pH (mid‐Goulburn), turbidity (upper and lower Murray and Yarra), nutrients (lower Murray and Yarra), and sedimentation rate (widespread), as well as clear shifts in trophic structure (upper Murray), have been documented for the post‐European period from regulated river wetlands across southeast Australia. A site in the lower Murray records river connectivity and water quality changes consistent with the regional Holocene climate record. Reductions in effective precipitation documented in closed lake systems are not evident in riverine plain wetlands, possibly owing to their relative complexity. The refinement of chronologies and data‐bases will allow the determination of the pre‐impact nature and variability of sites, the rates of limnological change and biological responses and the feasibility of rehabilitation targets. Copyright © 2005 John Wiley & Sons, Ltd.

Sedimentary geochemical evidence for recent eutrophication of Lake Chenghai, Yunnan, China

Geochemical anomalies and stable isotope ratios (δ18O, δ13C) in authigenic carbonates and organic matter (δ13C) from a 660-year sediment core from Lake Chenghai, southern China, provide a continuous history of recent lake eutrophication. The multi-proxy geochemical and isotopic record can be divided into a three-part history of contrasting limnological development, including: (1) a clear-water, oligotrophic open lake system (1340 and 1690 AD); (2) an environmentally unstable, hydrologically closed, oligotrophic lake system (1690–1940 AD); and (3) an increasingly eutrophic, closed lake system marked by higher organic matter, nitrogen, CaCO3, and pigment concentrations, and lower δ18O and δ13C values in authigenic calcite (1940–1999 AD). The unanticipated lowering of δ18O and δ13C of authigenic calcite during eutrophication is thought to be the result of disequilibrium water–carbonate fractionation of oxygen and carbon isotopes during periods of elevated primary production, pH, and [CO3 2−] activities in the water column. The recent eutrophication of Lake Chenghai indicated by these geochemical proxies is essentially simultaneous with large-scale human migration and the application of agricultural fertilizers in the catchment area during the 20th century.

Palaeoclimate interpretation of stable isotope data from lake sediment archives

The isotope composition of authigenic and biogenic carbonates and diatom silica are commonly used as palaeoclimate proxies from lake sediments. This article reviews the controls on the isotope composition of lacustrine skeletal and non-skeletal deposits and illustrates how stable isotope studies contribute to an understanding of changes in temperature, precipitation patterns, evaporation and the carbon cycle. It highlights the differences in the palaeoclimate potential of a wide range of lakes ranging from open to closed lake basins. A large number of the case histories, but not all, are drawn from studies of temperate lakes from Europe. Large closed lake systems, in the tropics and elsewhere, lose water predominantly through evaporation, and contain sediments with variable and generally high δ 18O values. Fluctuations in the isotope composition of authigenic or biogenic minerals are mainly a function of long-term changes in the precipitation/evaporation ratio. In contrast small open lakes which have a degree of through-flow typically contain sediments with δ 18O values that vary by no more than a few ‰. These variations are generally ascribed to variations in temperature or the isotope composition of precipitation (δ p), from which either an annual or seasonally specific signal can be gained. These types of lakes are common in Northern Europe and at high altitudes. The interpretation of isotope data from a lacustrine succession requires a knowledge of the local processes that might control and modify the signal. Their effects need to be quantified, and a robust calibration using the modern lake system is necessary to establish the relationship between the measured signal, the isotopic composition of the host waters, and climate.

Mineralogy and chemistry of bentonite (?) deposits at Minjingu, Lake Manyara, North Tanzania

Olive green clays likely to be bentonitic in composition have been mineralogically and chemically studied. They occur in association with other lacustrine sediments at Lake Manyara. Radiocarbon dates from four diatom horizons indicate ages ranging from 12 Ka to 135 Ka suggesting a Mid-Holocene age. Middle Pleistocene age have been assigned to the ridged oncolites of Lake Manyara. The olive green coloured clays in the Manyara basin are known to occur in association with other lake beds including phosphorite deposits, stromatolites, bioturbated silty clays, partly silicified marls, conglomerates and olive green coloured opal beds. The results presented herein are from the olive green coloured clays. The olive green clays (bentonite?) are a result of devitrification or alteration of volcanic ashes and/or pyroclasts. The green clays occur in different forms as they are separated from each other by other lacustrine sediments. The alteration might have taken place in slightly different environments in terms of salinity and alkalinity. One of the top layer is friable and shows conchoidal fractures when dry. The other beds below in the lacustrine sequence are cemented with calcite and some dolomite as well as zeolites. The lowermost layer in the sequence is friable and shows cracks filled with coarse crystalline calcite. Mineralogically the bentonite is composed of the clay minerals illite, illite-smectite mixed layer clays, and chlorite. Other authigenic minerals include various zeolites (analcime, clinoptilolite, erionite and some traces of mordenite), opal, and fluorapatite. The clays have magnesium contents varying from 3.01% to 7.43%. The calcium contents vary widely due to presence or absence of one of the two minerals calcite or apatite. Trace elements like Ba, Ce, Sr, Zr are equally attributed to the presence of calcite and apatite. The formation of the illite-smectite mixed layer clays in an alternating manner with other lake sediments depicts different episodes of volcanic eruptions in the area. The mineralogical composition of smectites, zeolites, and opal in the green clays suggests a deposition of pyroclasts and volcanic ashes in a closed lake system with fluctuating levels. Due to evaporation alkalinity and salinity levels were fluctuating. The clays might have been bentonite which have undergone illitisation, a phenomena noted in other neighbouring rift basins.

Terrestrial sediment yield and the lifetimes of reservoirs, lakes, and larger basins

Water reservoirs, lakes, and larger basins, including their drainage areas, represent sedimentologically closed to semi-closed denudation-accumulation systems. The mean rates of mechanical denudation, DRme, and clastic sedimentation, SRme, are related by the ratio of the drainage/lake area, Ad/Al. If the latter is known, DRme (or the specific sediment yield SY in t per km2/a) can be calculated from SRme, or vice versa. The best data for modern SY mainly come from the sediment fills of artificial reservoirs. Small drainage areas of mountainous regions show SY values up to two orders of magnitude higher than lowlands and approximately one order higher than larger regions of mixed relief. This is also true of arid to semi-arid zones which often provide approximately as much sediment (SY) as humid temperate and even tropical zones of comparable relief. Lithology and climate (river runoff) also may play some role for SY from catchments of limited size. The importance of these factors is exemplified by perialpine lakes and two East African lakes. Sediment yields gained from some large reservoirs compare well with long-term denudation rates derived from geological studies (e.g., the Tarbela dam reservoir along the Indus River). In many other cases, human activities have raised SY by factors of 2–10, locally up to >100. Artificial reservoirs in mountainous regions with SY in the range of 300–2000 t per km2/a tend to become filled within several tens to hundreds of years; some have even shorter lifetimes. Perialpine lakes of the Alps and British Columbia are strongly affected by delta prograding and have lifetimes mostly between 15 and 40 ka. Closed lake systems in deep morphological depressions (Lake Bonneville, Aral Sea, northern Caspian Sea) have a high potential for sediment storage up to the level of spillover and therefore can persist over long time periods. Basins with markedly subsiding basin floors (lakes of the East African rift zone, the southern Caspian Sea, and the Black Sea, both on oceanic crust) can survive for many Ma in the future, despite relatively high terrigenous input.

Sources and Accumulation of Sedimentary Organic Matter in a Closed Lake System: Pyramid Lake, Nevada: ABSTRACT

Pyramid Lake is a saline, terminal lake that has experienced a variety of changes in its aquatic and watershed environments since arrival of European settlers in the mid-1800s. We have examined the organic matter contents of surficial sediments from 32 lake sites and in two [sup 210]Pb-dated sediment cores to determine their sources and delivery of organic matter. The primary influence on sediment in the southern basin is input from land-derived clastic components by the river. Concentrations of CaCO[sub 3] and total organic carbon, mass accumulation rates of CaCO[sub 3] and total organic carbon, and isotopic compositions of carbonate carbon, and oxygen increase with increasing distance from the mouth of the Truckee River, whereas organic C/N atomic ratios and carbon isotopic compositions decrease Organic C/N ratios, [delta][sup 13]C values, and terrigenous/aquatic n-alkane ratios indicate that the proportion of land-derived organic matter in sediments has decreased as agricultural diversion of Truckee River has increased. Elevations in the proportion of land-plant wax and in organic C/N ratios in sediments deposited tram 1200-1400 AD suggest that contributions of land-derived organic matter were enhanced at this time, probably because of an interval of regionally wetter climate.